1. Field of the Invention
The present invention relates to a high toughness amorphous alloy strip having a large thickness and a process of producing the strip using a single-roll liquid-quenching method.
2. Description of the Related Art
There are known methods for continuously producing strips by quenching a molten metal alloy, such as centrifugal quenching methods, single roll methods, and twin-roll methods. These methods include supplying a molten metal, through an orifice, onto an inner or outer circumferential surface of a rapidly rotating metal drum or roll, thereby rapidly solidifying the molten metal to form a strip or wire. Proper selection of alloy composition provides an amorphous metal alloy having a structure like a liquid metal.
Amorphous alloys are drawing attention for their characteristic properties and some of them are practically used. However, to establish an amorphous structure, a molten metal must be cooled at a high cooling or solidification rate, so that the product strip generally has a small thickness. This restricts the application of amorphous metal alloys.
It is known that the maximum possible thickness of amorphous metal alloys depends on the alloy composition. Hagiwara et al. obtained a 250 xcexcm thick, 1 mm wide strip of an Fexe2x80x94Sixe2x80x94B alloy by using a single roll method, which is a single side cooling method (Sci. Rep. Res. Inst. Tohoku Univ. A-29(1981), 351). However, it is empirically known that such a thick strip cannot be obtained with a practically available strip width of 20 mm or more. In the conventional single roll method, it is generally believed that the strip thickness can be varied by the manufacturing parameters of (1) the width of nozzle opening measured in the roll rotation direction, (2) the ejection pressure of molten metal, (3) the distance between the nozzle and the roll surface, and (4) the circumferential speed of roll. However, merely varying these parameters did not provide a strip thickness more than 45 xcexcm when the strip width is 25 mm. If a thicker strip is forcibly produced with these parameters outside proper ranges, the strip has poor shape, surface, magnetic, and mechanical properties. It is thus extremely difficult to produce a practically available wide and thick amorphous metal alloy strip.
J. Appl. Phys., Vol. 55, 1787 (1984) reported that a 25.4 mm wide, 80 xcexcm thick strip was obtained. An as-quenched strip of an Fe80B14.5Si3.5C2 alloy had a bending fracture strain xcex5f which was reduced, with an increase of the strip thickness, to as small as 0.01 when the strip thickness was 40 xcexcm or more. The fracture strain xcex5f is defined by xcex5f=t/(Dxe2x88x92t), where t represents the strip thickness and D represents the bending radius upon fracture. For example, when a 60 xcexcm thick strip has a fracture strain of 0.01, this means that the strip cannot be coiled around a cylinder having a diameter of 6 mm or less. This causes a drawback in fabricating a coiled core of transformers, not only because the radius of bending the strip at the corners of the coil is limited but also because breakage of the strip being coiled frequently occurs.
To reduce the occurrence of cracking during blanking an amorphous alloy strip, Japanese Unexamined Patent Publication (Kokai) No. 61-153266 disclosed a process in which a solidified strip is detached or separated from the cooling roll surface at a position distant from a molten metal application point, the distance being the longer of xc2xc of the roll circumferential length and 10 cm. This publication, however, describes nothing about the strip temperature and moreover, the strips provided as examples are made of Co-based amorphous alloys and have a thickness of as small as 30 xcexcm.
Japanese Unexamined Patent Publication (Kokai) No. 56-33443 disclosed a process in which a solidified strip is detached from the cooling roll surface at a position 30 cm or more distant from the molten metal application point. This publication also describes nothing about the strip temperature and is limitedly directed to Co-based amorphous alloy strips. Moreover, the strips provided as examples have a thickness as small as 27 xcexcm and there is no reference to the mechanical property.
Thus, there is no conventional amorphous alloy strip which is not only thick but also excellent in the mechanical property and the appearance has been desired.
To provide a solution to this problem, the present inventors have found a process for producing an amorphous alloy strip having a high toughness and a large thickness.
According to Japanese Unexamined Patent Publication (Kokai) No. 60-255243, a multiple slot nozzle is used to produce a quenched strip of an Fe-based amorphous alloy having a large thickness and a high toughness, i.e., a 50 xcexcm or thicker, 20 mm or wider strip having a bending fracture strain of 0.01 or more as determined by bending the strip with the free cooling surface facing outward. According to Japanese Unexamined Patent Publication (Kokai) No. 61-212449, the strip has a further improved toughness when the strip is quenched at a cooling rate of 103xc2x0 C./sec or more, determined at the free surface thereof, in a temperature range of from 500 to 300xc2x0 C. and the thus-solidified strip is detached from a cooling substrate at a temperature of 300xc2x0 C. or less.
Thus, Japanese Unexamined Patent Publication (Kokai) Nos. 60-255243 and 61-212449 disclosed amorphous alloy strips having an increased thickness and an improved toughness.
However, it is more desirable from practical point of view if the bending fracture strain xcex5f equals 1 irrespective of the strip thickness so that the strip can be bent completely onto itself to form a folio. For mass production of amorphous alloy strips to be brought into practice, it is necessary to continuously coil the quenched strips. If the coiling temperature is not controlled as in the conventional processes, it is not ensured to produce a strip having a bending fracture strain xcex5f of 1, because the heat of the coiled strip is accumulated in the coil and deteriorates the magnetic and mechanical properties of the strip.
Japanese Unexamined Patent Publication (Kokai) Nos. 60-255243 and 61-212449, in the Examples thereof, describe that xcex5f is less than 1 when the strip thickness is more than 55 xcexcm and the strip unavoidably breaks when used in applications in which the strip must be bent with a small radius. Moreover, when a wide amorphous strip is slit to a narrower width, it was conventionally difficult to avoid breaking of the strip being slitted when the strip is 55 xcexcm or thicker, irrespective of the process by which the strip was produced. It should be also noted that Japanese Unexamined Patent Publication (Kokai) Nos. 60-255243 and 61-212449 consider nothing about the heat accumulation in a coil and that Japanese Unexamined Patent Publication (Kokai) No. 61-212449 merely controls the temperature at which the solidified strip is detached from the cooling substrate, so that the strip is unavoidably embrittled when continuously coiled.
It is desirable that the above-mentioned drawback in a thick amorphous alloy strip is eliminated and that a high toughness, thick amorphous alloy strip is developed.
As described above, no conventional thick and wide amorphous alloy strip having a good mechanical property, even when continuously coiled in mass production, is available and no conventionall process of producing such a strip has been developed.
It is the object of the present invention to provide a thick, wide, and high toughness amorphous alloy strip, and a process of producing same, the strip having good mechanical properties, particularly a toughness sufficient for slitting requiring a bending fracture of 1 so that the amorphous alloy strip can be used to fabricate transformer cores or the like without difficulty.
To achieve the object according to the present invention, there is provided a process of producing a high toughness iron-based amorphous alloy strip, using a single roll liquid quenching method, the strip having a thickness of more than 55 xcexcm up to 100 xcexcm and a width of 20 mm or more and having a fracture strain xcex5f satisfying the following relationship:
xcex5f greater than 0.1
where xcex5f=t/(Dxe2x88x92t),
t=thickness of the strip, and
D=bent diameter upon fracture,
xcex5f being determined by bending the strip with a free cooling surface thereof facing outward, the process comprising the steps of:
ejecting a molten metal alloy through a nozzle;
applying the ejected molten metal alloy to a surface of a rotating roll;
allowing the applied molten metal alloy to be quenched by the roll surface to form an amorphous strip of the metal alloy, the strip being quenched at a cooling rate, determined at a free surface thereof, of 103xc2x0 C./sec or more in a temperature range of from 500xc2x0 C. to 200xc2x0 C.; and
continuously coiling the quenched strip at a temperature of 200xc2x0 C. or lower.
Preferably, the strip has a thickness of more than 55 xcexcm up to 70 xcexcm and the fracture strain xcex5f satisfies the following relationship:
xcex5f=1.
According to the present invention, there is also provided a high toughness iron-based amorphous alloy strip having a thickness of more than 55 xcexcm up to 100 xcexcm and a width of 20 mm or more and has a fracture strain xcex5f satisfying the following relationship:
xcex5f greater than 0.1,
where xcex5f=t/(Dxe2x88x92t),
t=thickness of the strip, and
D=bent diameter upon fracture,
xcex5f being determined by bending the strip with a free cooling surface thereof facing outward.